But not all Bt hybrids scored knockouts when compared with conventional hybrids, especially when the latter were sprayed with an insecticide. Crop consultants and extension specialists point out that there's still lots of yield variability among insect-resistant hybrids, though they're getting better.

"We saw especially heavy corn borer pressure in Nebraska and northern Illinois and more moderate activity in Iowa, western Indiana and southern Wisconsin," Milner reports. "But corn borer numbers were at or above economic threshold levels in all the areas we cover, from eastern Nebraska to Ohio."

What's more, says Milner, there are indications that both univoltine (one generation per year) and multivoltine (two or more generations per year) corn borers are active in parts of the Corn Belt.

"In the past four years we have seen extended flights. We are not getting the distinct break between flights that you would expect with just the multivoltine borers."

Universities, seed companies, crop consultants and producers conducted wide- scale testing of Bt hybrids in 1997. Here are some of the box scores:

University of Minnesota scientists conducted trials at three southern Minnesota locations, where late-season corn borer pressure was quite heavy. Bt hybrids (excluding one that does not have full-season Bt protection) averaged 15, 11.4 and 11.8 bu/acre more than their non-Bt counterparts.

At three other trial sites, farther north in Minnesota where there was heavy mid- to late-season borer pressure, Bt hybrids averaged 23.8 bu/acre higher than non-Bts. But Bt hybrids without full-season protection had poorer control than those with it at both areas, Ostlie reports.

University of Illinois entomologists conducted a trial with 22 Bt and non-Bt hybrids. They infested the plot with first- and second-generation corn borer egg masses, applying an insecticide to some non-Bt hybrids for both borer generations.

The two highest-yielding hybrids, at 200.8 and 194.9 bu/acre, were Bts. However, the third-highest yielder, at 183.2 bu, was a non-Bt hybrid treated with an insecticide. Among the remaining hybrids, there was no superiority for Bt hybrids over insect-treated non-Bts.

In Ohio State University trials in northwestern Ohio, with borers at economic threshold levels, Bt hybrid yields generally were not statistically different from their unsprayed non-Bt counterparts.

Pioneer trials in Minnesota and the Dakotas showed that some Bt hybrids yielded at least 30 bu better than anything else in heavily infested areas.

"With Pioneer hybrids that have the YieldGard gene for protection against European corn borers, you could see a notable difference, especially under the high pressures of last summer," says company field agronomist Kevin Brannick, Sioux Falls, SD.

According to a Monsanto report, studies at nearly 200 locations across the Corn Belt demonstrated that Bt hybrids with the YieldGard gene outperformed non-Bt hybrids by an average of 14.9 bu/acre. The non-Bts were identical to the Bts except for the resistance gene.

Monsanto officials say that difference translates into a $27.25 per acre return after paying for the Bt technology.

In 72 locations last summer, Mycogen trials showed a 13.6-bu yield advantage for a new Bt hybrid compared to its non-Bt counterpart. That was under varying levels of corn borer pressure.

Gary Schneider, a grower from Streator, IL, had a no-till test plot with 35 hybrids, 13 of which were Bts. He did no insecticide spraying.

The top performer (yield less drying costs) was a Bt hybrid that yielded 195.6 bu/acre. The second-best performer was a non-Bt hybrid yielding 177.5 bu.

Bt hybrids finished third, fifth and sixth, too, but also came in 27th, 28th, 29th and 32nd.

Schneider says there was stalk rot in most of the hybrids, both Bt and non-Bt. There also have been other reports of stalk rot in Bt corn.

However, University of Illinois plant pathologist Walker Kirby points out that stalk rot can enter plants in various ways other than through openings created by corn borers. He also points out that 1997 was a prime year for stalk rot in many areas.